The nature of spacetime in bigravity: Two metrics or none?

Akrami, Yashar; Koivisto, Tomi S.; Solomon, Adam R.

doi:10.1007/s10714-014-1838-4

Cited by 67 publications

(78 citation statements)

References 40 publications

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“…Refs [121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137] for generalized couplings to matter, Ref. [138] with mass parameters promoted to Lorentz-invariant functions of the Stückelberg fields, and Refs.…”

Section: Gravity In Cosmologymentioning

confidence: 99%

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

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459

210

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Despite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, ΛCDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of ΛCDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled.Keywords: cosmology -dark energy -cosmological constant problem -modified gravitydark matter -early universe Cosmology has been both blessed and cursed by the establishment of a standard model: ΛCDM. On the one hand, the model has turned out to be extremely predictive, explanatory, and observationally robust, providing us with a substantial understanding of the formation of large-scale structure, the state of the early Universe, and the cosmic abundance of different types of matter and energy. It has also survived an impressive battery of precision observational tests -anomalies are few and far between, and their significance is contentious where they do arise -and its predictions are continually being vindicated through the discovery of new effects (B-mode polarization [1] and lensing [2,3] of the cosmic microwave background (CMB), and the kinetic Sunyaev-Zel'dovich effect [4] being some recent examples). These are the hallmarks of a good and valuable physical theory.On the other hand, the model suffers from profound theoretical difficulties. The two largest contributions to the energy content at late times -cold dark matter (CDM) and the cosmological constant (Λ) -have entirely mysterious physical origins. CDM has so far evaded direct detection by laboratory experiments, and so the particle field responsible for it -presumably a manifestation of "beyond the standard model" particle physics -is unknown. Curious discrepancies also appear to exist between the predicted clustering properties of CDM on small scales and observations. The cosmological constant is even more puzzling, giving rise to quite simply the biggest problem in all of fundamental physics: the question of why Λ appears to take such an unnatural value [5,6,7]. Inflation, the theory of the very early Universe, has also been criticized for being fine-tuned and under-predictive [8], and appears to leave many problems either unsolved or fundamentally unresolvable. These problems are indicative of a crisis.From January 14th-17th 2015, we held a conference in Oslo, Norway to surve...

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Section: Gravity In Cosmologymentioning

confidence: 99%

BeyondΛCDM: Problems, solutions, and the road ahead

Bull

Akrami

Adamek

et al. 2016

Physics of the Dark Universe

Self Cite

459

210

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“…In the simpler cases of massive gravity and bigravity, where there are two metrics rather than three, the question of how matter couples was the source of much discussion and debate [20,29,47,68,[70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85], leading to the conclusion that the Boulware-Deser ghost almost always re-emerges if any matter field couples to more than one metric, or if matter coupled to one metric interacts with matter coupled to another. 4 We will therefore take all matter to couple minimally to a single metric, which we will call g. Because matter moves on geodesics of this metric, we can interpret it as the physical metric describing the geometry of spacetime, exactly like in general relativity.…”

Section: The Theory Of Massive Trigravitymentioning

confidence: 99%

Cosmology with three interacting spin-2 fields

et al. 2016

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Theories of massive gravity with one or two dynamical metrics generically lack stable and observationally-viable cosmological solutions that are distinguishable from ΛCDM. We consider an extension to trimetric gravity, with three interacting spin-2 fields which are not plagued by the Boulware-Deser ghost. We systematically explore every combination with two free parameters in search of background cosmologies that are competitive with ΛCDM. For each case we determine whether the expansion history satisfies viability criteria, and whether or not it contains beyond-ΛCDM phenomenology. Among the many models we consider, there are only three cases that seem to be both viable and distinguishable from standard cosmology. One of the models has only one free parameter and displays a crossing from above to below the phantom divide. The other two provide scaling behavior, although they contain future singularities that need to be studied in more detail. These models possess interesting features that make them compelling targets for a full comparison to observations of both cosmological expansion history and structure formation.

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“…(21,22) is due to the fact that here we suppose that there exists some (sufficiently large) region of space within which we may neglect the spatial derivatives of φ at some initial time τ i , explicitly…”

Section: A General Settingmentioning

confidence: 99%

Inflationary perturbations in bimetric gravity

Cusin¹,

Durrer²,

Guarato³

et al. 2015

J. Cosmol. Astropart. Phys.

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In this paper we study the generation of primordial perturbations in a cosmological setting of bigravity during inflation. We consider a model of bigravity which can reproduce the ΛCDM background and large scale structure and a simple model of inflation with a single scalar field and a quadratic potential. Reheating is implemented with a toy-model in which the energy density of the inflaton is entirely dissipated into radiation. We present analytic and numerical results for the evolution of primordial perturbations in this cosmological setting. We find that the amplitude of tensor perturbations generated during inflation is sufficiently suppressed to avoid the effects of the tensor instability discovered in Refs. [1,2] which develops during the cosmological evolution in the physical sector. We argue that from a pure analysis of the tensor perturbations this bigravity model is compatible with present observations. However, we derive rather stringent limits on inflation from the vector and scalar sectors.

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The nature of spacetime in bigravity: Two metrics or none?

Cited by 67 publications

References 40 publications

BeyondΛCDM: Problems, solutions, and the road ahead

BeyondΛCDM: Problems, solutions, and the road ahead

Cosmology with three interacting spin-2 fields

Inflationary perturbations in bimetric gravity

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